Multishaft Turbofan Engines
An arrangement of this type of engine is suggested by the sketch of Fig.
1.26. A photograph of a production version for subsonic flight is shown in
Fig. 1.27. Recall that the fan is nothing more than a compressor producing
a relatively low pressure ratio. The off-design behavior of the components
of this engine is similar to the two-shaft turbojet, since the turbines and jet
divide the pressure ratio in about the same way. Observe that when the
ETR is lowered, the power to the jet diminishes faster than that to the fan.
The discussion of turbofan engines in Sec. 1.3 demonstrated that there is an
optimum division of these powers, which depends on flight speed and
internal efficiencies. Prolonged operation at two or more flight conditions
requires the power division to be compromised unless some variable feature
is provided.
Another engine currently in production uses three concentric shafts: there
are two sets of compressor and turbine rotors and a turbine on the third
shaft drives the fan. The thrust nozzle is downstream of the third turbine.
Reducing the ETR first causes the nozzle to unchoke; the fan turbine
becomes unchoked at a slightly lower ETR. The changes in the ratio of fan
power to jet power is different for this design and a different off-design
efficiency is thus expected.
Still another concept is shown in Fig. 1.28. The objective of the design is
to put the turbine driving the fan immediately downstream of the highpressure
turbine; the tortuous flow path was accepted in order to achieve
this objective. This arrangement was a deliberate attempt to provide a
favorable balance of fan and jet power at low levels of specific engine
power, while still maintaining acceptable engine efficiencies. The aerodynamic
aims were almost realized in spite of the intricate flow path. There
was an unfortunate mechanical problem, however. The regions of high
pressure within the engine are on the right side of the photograph for both
the compressors and turbines. A large pneumatic force, which could not be
accommodated by rolling element thrust bearings, is exerted toward the left
on the rotors. Pressurized air bearings requiring rotating air seals having
large diameters are necessary. Precise control of clearances is mandatory to
prevent the associated leakage losses from overwhelming the gains; thus,
the costs of manufacturing and maintenance control the value of this
design. This is a good illustration of a need for close cooperation among
aerodynamic, mechanical design, and manufacturing specialists early in a
program.
An arrangement of this type of engine is suggested by the sketch of Fig.
1.26. A photograph of a production version for subsonic flight is shown in
Fig. 1.27. Recall that the fan is nothing more than a compressor producing
a relatively low pressure ratio. The off-design behavior of the components
of this engine is similar to the two-shaft turbojet, since the turbines and jet
divide the pressure ratio in about the same way. Observe that when the
ETR is lowered, the power to the jet diminishes faster than that to the fan.
The discussion of turbofan engines in Sec. 1.3 demonstrated that there is an
optimum division of these powers, which depends on flight speed and
internal efficiencies. Prolonged operation at two or more flight conditions
requires the power division to be compromised unless some variable feature
is provided.
Another engine currently in production uses three concentric shafts: there
are two sets of compressor and turbine rotors and a turbine on the third
shaft drives the fan. The thrust nozzle is downstream of the third turbine.
Reducing the ETR first causes the nozzle to unchoke; the fan turbine
becomes unchoked at a slightly lower ETR. The changes in the ratio of fan
power to jet power is different for this design and a different off-design
efficiency is thus expected.
Still another concept is shown in Fig. 1.28. The objective of the design is
to put the turbine driving the fan immediately downstream of the highpressure
turbine; the tortuous flow path was accepted in order to achieve
this objective. This arrangement was a deliberate attempt to provide a
favorable balance of fan and jet power at low levels of specific engine
power, while still maintaining acceptable engine efficiencies. The aerodynamic
aims were almost realized in spite of the intricate flow path. There
was an unfortunate mechanical problem, however. The regions of high
pressure within the engine are on the right side of the photograph for both
the compressors and turbines. A large pneumatic force, which could not be
accommodated by rolling element thrust bearings, is exerted toward the left
on the rotors. Pressurized air bearings requiring rotating air seals having
large diameters are necessary. Precise control of clearances is mandatory to
prevent the associated leakage losses from overwhelming the gains; thus,
the costs of manufacturing and maintenance control the value of this
design. This is a good illustration of a need for close cooperation among
aerodynamic, mechanical design, and manufacturing specialists early in a
program.
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